When repairing a broken, fractured, or shattered bone, a physician may often be faced with the task of affixing a fixation plate to the bone in order to align the bone, and possibly, to hold bone fragments together. In order to affix the fixation plate to the bone, a surgeon may insert a locking bone screw through one of a plurality of threaded screw holes in the fixation plate and into a predrilled hole in the bone. Alternatively, self-drilling screws may be used. Since numerous threaded screw holes may be spread out across the entirety of the fixation plate, the surgeon may affix virtually any portion of the fixation plate to the bone by inserting a suitable number of locking bone screws through the plate and into the bone.
The trend in fixation for many medical practice areas such as the small bone orthopedic market and the craniofacial market is to use locking screws and plates that prevent the locking bone screws from backing out of the fixation plate once inserted. To achieve this lockable engagement, the inner surface of each threaded screw hole may be threaded to engage a corresponding set of locking threads on the head of each locking bone screw. Consequently, as a locking bone screw is screwed into one of the threaded screw holes in the fixation plate, the locking threads in the screw hole and the locking threads on the head of the locking bone screw may deform against each other to lock the locking bone screw into the fixation plate.
In certain cases, proper placement and positioning of the fixation plate may call for inserting a locking screw into a threaded screw hole at an angle other than perpendicular to the central axis of the threaded screw hole. For example, if the underlying bone beneath a particular screw hole is weak due to its proximity to a fracture line, the surgeon may wish to angle the bone screw away from the fracture line so as to anchor the screw into a more solid bony mass. Consequently, the ability to lockably engage a bone screw into a fixation plate at an angle off of perpendicular from the plate maybe a desirable feature for a surgeon repairing a broken, fractured, or shattered bone.